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A closely watched and extremely expensive special congressional election in Pennsylvania’s 18th Congressional District remained too close to call late Tuesday.

Such a tight margin will likely be seen as a major victory for the Democrat Conor Lamb, as President Donald Trump won the area in southwest Pennsylvania by nearly 20 points during the 2016 election cycle and in the last two cycles the Republican incumbent ran unopposed.

At 11:30 P.M. eastern time, with nearly all precincts reporting and most of the votes in, Lamb and Republican Rick Saccone were separated by less than 1,000 votes, with the Democrat holding on to a narrow lead.

Despite the fact that the Associated Press did not call the race on Tuesday night, the Democratic Congressional Campaign Committee declared victory.

"These results should terrify Republicans," DCCC Chairman Rep. Ben Ray Luján of New Mexico said in a statement.

Saccone vowed to fight on.

"We're going to fight all the way to the end. You know I never give up," Saccone said at his election headquarters on Tuesday night.

His Democratic opponent, however, declared victory.

"It took a little longer than we thought, but we did it," Lamb said at the top of his remarks early Wednesday morning.

Just a minute earlier, he was announced to the stage as "Congressman-elect Conor Lamb."

The final result may come down absentee ballots that have been returned but not tabulated yet.

There are two remaining counties in the district with absentee votes left to be tallied.

Washington County also expects to count their ballots late Tuesday night into Wednesday morning.

Greene County will not count their absentee ballots until tomorrow.

With the final result still unclear, the possibility of a recount looms.

While state law stipulates that an automatic recount is triggered when the margin in a statewide race is less than half a percentage point, the rules for a special election in a congressional district remain unclear.

Jeff Swensen/Getty Images

Campaign volunteer Chloe Chappell checks early election results at an Election Night event for GOP PA Congressional Candidate Rick Saccone as the polls close on March 13, 2018 in Elizabeth Township, Pennsylvania.

The results give the Democratic Party a boost in confidence and shores up their belief that their candidates can run and compete in any House district in the country — even the reddest ones. President Donald Trump took the district by 20 points in 2016.

"Democrats have now expanded the playing field, and they're going to compete in districts Republicans won by 12, 14, 16 points," ABC News contributor and political analyst Matt Dowd said Tuesday night.

They’ll also argue their base is much more motivated than the GOP voters who gave Trump the White House.

The numbers appear to back that up.

Turnout in the most Democratic county, Allegheny, was 6 percentage points higher than the other three more Republican counties that make up the 18th Congressional District.

At this moment, Conor Lamb is within a whisker of beating Rick Saccone in a Pennsylvania congressional district Trump won by double digits. Regardless of the outcome, R's in Congress should know their days are numbered. Some call it a blue wave. I call it a sane wave.

But, a bigger part of the math that made this race so tight was that Lamb also seems to have swung votes and outperformed Hillary Clinton across the board, doing 10 percentage points better than the 2016 Democratic presidential nominee.

Lamb, a 33-year old Marine Corps veteran and former assistant U.S. attorney has been locked in a tight battle with Saccone, a 60-year old, four-term state representative to represent the southwestern Pennsylvania district.

Tuesday’s special election was triggered by the resignation of Republican Rep. Tim Murphy last year following a report that he allegedly asked an extramarital lover to end her pregnancy.

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GOP candidate Rick Saccone: 'We're still fighting the fight'

The closely-watched contest has seen close to $10 million spent by conservative groups to boost Saccone and stave off concerns that the GOP is falling out of favor with the same blue-collar voters that propelled Donald Trump to the White House.